This invention generally relates to the field of injection molding. More specifically, the present invention relates to the reduction of galling between mold components as they experience rotation relative to one another. In yet a further specific aspect, the present invention describes an improved method and apparatus for the manufacture of articles having internally disposed threads in which a thread-forming core is at least partially disengaged from the molded article under substantially full mold clamp pressure.
The art of forming internally threaded plastic injection molded closures is generally well known in the industry. Injection molds of this type typically include a female mold housing which substantially surrounds at least a partially threaded core component. The mold cavity is generally defined by the void space between a female mold housing and an internally disposed core component. Molten plastic material is usually injected into the mold cavity to form the threaded article. After injection and molding of the plastic, coolant may then be introduced to circulate through channels in various mold components in order to accelerate cooling of the product article. Once the article has cooled, a general feature of injection mold systems is the application of an ejection mechanism for removal of the article. Because a plastic injection mold generally has a plurality of mold cavities, it is often generally the case that the ejection mechanism operates to dislodge the articles in a group for each product cycle of the mold.
In the case of prior art methods of forming and ejecting threaded, molded products, the female mold half and mold core half are separated to initiate removal of the article from the mold. Because of the engaging nature of the threads however, the article generally remains connected to the face of the stripper ring upon separation of the mold halves. Accordingly, an ejection mechanism is generally required for subsequent removal of the article from the outer surface of the mold core.
Depending on the design parameters of the thread-molded article, the product may be removed from the mold core in various ways. These parameters may vary according to the type of plastic used to form the article as well as the number and type of threads to be formed. If the molded article is flexible, and the thread type permits, the article may be removed from the threaded core by the action of a stripper ring. In this process, the plastic should be sufficiently resilient and elastic to return to its original conformation, within a specified tolerance, after the formed threads have been stretched over the threaded core during extraction. If the polymer material is not flexible, or if the thread profile is very deep, very thin or has a more cantilevered shape, stripping may damage the article. An additional complication may occur when the thread-molded product has inherently delicate features, such as a tamper evident ring, which may experience strip-ejection damage even if an otherwise suitable polymer were to be used.
Additional prior art methods and devices for removing internally threaded articles from a mold include, for example, separation of the mold halves prior to disengagement of the article from the threaded mold core. These methods generally involve first separating the mold halves and then rotating the threaded-core while a stripper grabber ring engages the molded article and translates axially along the core in timed relation to the rotation and pitch of the threaded core. In this regard, the stripper ring may often have structural features known as grabbers to hold the molded article and prevent it from turning with the rotation of the threaded core. Such methods generally known in the art, however, have previously been applied to mold timing cycles where rotational removal of the article is accomplished only after the article has suitably cooled and the mold halves have been separated. For example, in U.S. Pat. No. 5,421,717 to Hynds, incorporated herein by reference, a moveable ejection mechanism, including a camming mechanism, which engages a stripper ring, is used to remove the article from the mold in an open-clamp configuration after the mold halves are separated.
On the other hand, U.S. Pat. No. 4,130,264 to Schroer, incorporated herein by reference, discloses an apparatus in which a plurality of thread-forming components are peripherally disposed around the core which translate on tracks to cause the core to collapse so that the thread-molded article may be pushed off. However, the collapse and expansion of the core in this device adds substantially to the overall complexity and cost of the injection mold apparatus as well as the production cycle time between mold injections. Additionally, the collapse of the core is typically engaged only after the additional step of separating the mold halves.
In the case of the manufacture of a tamper evident ring, U.S. Pat. No. 4,155,698 to Aichinger, incorporated herein by reference, generally discloses a device in which a first female cavity component surrounds a threaded component and is removed from the molded closure while a second female component adjacent to the tamper evident ring remains in place. However, this method, while generally effective, is uniquely adapted for the production of molded caps having an integral tamper evident ring and also typically includes separation of the mold halves prior to disengagement of the article.
Alternatively, when using a polymer which is generally too inflexible to be ejected by the action of a stripper ring without permanent stripping damage to the article, a method disclosed in U.S. Pat. No. 4,625,227 to Hara, incorporated herein by reference, may be used. In the ""227 patent to Hara, a rotationally displaced chuck is engaged over the molded article after the female component of the mold cavity is removed. The chuck engages the outer edge of the closure and rotates the closure as it translates backward to allow the rotational removal of the unscrewing article. This method, however, is often applied in mold timing cycles where the mold is separated prior to rotational removal of the article.
Thus, a need exists in the injection molding art for a method and apparatus for the molding and ejection of threaded articles in which the injection cycle time is substantially reduced while simultaneously preserving the thread integrity of the articles. As such, the need exists for a device capable of realizing a reduced in-mold product cooling time, the commencement of resolved rotational disengagement of the article from the threaded mold core under substantially full mold clamp pressure, and the achievement of a greater number of injection production cycles between periodic inspection and maintenance checks.
The present invention generally relates to the production and removal of threaded, molded articles from a plastic injection mold device. Articles having internally disposed threads are created by a thread-forming core, which may be rotationally disengaged from the article under substantially full mold clamp pressure. A cam system and linear drive/following gear mechanism are employed to engage a finely resolved retraction of the threaded core under substantially full mold clamp pressure prior to substantially complete rotational disengagement of the threaded core from the product article and subsequent separation of the mold halves.
Specifically, the mold halves are brought together to a closed-mold position to create a mold cavity for receiving molten plastic with the core in the set position. As plastic is injected into the mold, the liquid plastic fills the cavity to form the product part. The product part may then be partially cooled in preparation for removal from the mold. Thereafter, a linear drive system is engaged to partially retract the threaded core away from the metal-to-metal contact areas of the shutoffs under substantially full mold clamp pressure. After the threaded core is subsequently disengaged from the product part, still under substantially full mold clamp pressure, the mold halves are opened to expose the part for ejection from the mold. The molded part is then ejected, the mold halves are returned to a closed position, the cores are re-set and the mold is readied for the next production cycle. While the timing and order of these steps may be varied, many of the steps may occur substantially simultaneously at various points in the mold cycle, to reduce or otherwise optimize the production cycle time.
The present invention is additionally directed to reducing galling that may otherwise occur when mold components experience rotation with respect to each other without initial retraction of the core under pressure in closed-mold configurations. Moreover, the need for periodic maintenance and incident interruption of production is substantially reduced as well.